JP7206792B2 - Steel for line pipes - Google Patents

Description

TECHNICAL FIELD The present invention relates to steel materials, and more particularly to steel materials for line pipes.

Pipelines laid on the sea bed pass high-pressure fluids through them. Pipelines are also subjected to cyclic strain due to waves and sea pressure. Therefore, steel pipes used for undersea pipelines are required to have high strength and high low-temperature toughness.

A pipeline is composed of a plurality of line pipes. Electric resistance welded steel pipes (hereinafter referred to as electric resistance welded steel pipes) are sometimes used as steel pipes for line pipes. High strength can be obtained by increasing the wall thickness of the electric resistance welded steel pipe. However, if the wall thickness increases, brittle fracture is likely to occur and toughness decreases. Therefore, thick-walled electric resistance welded steel pipes are required to have excellent toughness.

As an index of low temperature toughness, there is a DWTT (Drop Weight Tear Test) guaranteed temperature. The DWTT guarantee temperature means the temperature at which the DWTT test has a ductile fracture rate of 85% or more. The lower the DWTT guaranteed temperature, the higher the low temperature toughness. In recent years, electric resistance welded steel pipes for line pipes are required to have better low-temperature toughness than ever before.

International Publication No. 2012/002481 (Patent Document 1) proposes a manufacturing method for increasing the low temperature toughness of hot-rolled steel sheets for line pipes.

The hot-rolled steel sheet for line pipes disclosed in Patent Document 1 has C = 0.02 to 0.08%, Si = 0.05 to 0.5%, Mn = 1 to 2%, Nb = 0.03 to 0.12%, Ti = 0.005 to 0.05%, and the balance consists of Fe and unavoidable impurity elements. In the microstructure at a depth of 1/2 of the plate thickness from the surface of the steel plate, the proeutectoid ferrite fraction is 3% or more and 20% or less, the others are the low temperature transformation phase and 1% or less pearlite, and the number of the entire microstructure The average crystal grain size is 1 μm or more and 2.5 μm or less, the area average grain size is 3 μm or more and 9 μm or less, the standard deviation of the area average grain size is 0.8 μm or more and 2.3 μm or less, and the thickness from the steel plate surface The reflected X-ray intensity ratio {211}/{111} in the {211} direction and the {111} direction with respect to a plane parallel to the surface of the steel sheet at a depth of 1/2 thickness is 1.1 or more. Patent Document 1 states that this steel plate for line pipes can obtain excellent strength and low-temperature toughness by controlling the proeutectoid ferrite fraction at the center of the thickness, the average grain size, and the texture. It is

WO2012/002481

However, in the hot-rolled steel sheet disclosed in Patent Document 1, the heating temperature before the rolling process is high, and the austenite grains may become coarse. In this case, the crystal grains may become coarse and the low temperature toughness may deteriorate.

An object of the present invention is to provide a steel material for line pipes having excellent low temperature toughness.

The steel material for line pipes according to the present embodiment has, in mass %, C: 0.010 to 0.060%, Si: 0.05 to 0.30%, Mn: 0.50 to 2.00%, P: 0 ~0.030%, S: 0-0.0100%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010-0.080%, Ti: 0.005-0.030%, Ni: 0.001-0.50%, Mo: 0.05-0.30%, O: 0-0.0030%, Ca: 0-0.0050%, V : 0 to 0.100%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, and a rare earth element: 0 to 0.0100%, The remainder consists of Fe and impurities, and has a chemical composition that satisfies formula (1). In the structure at the center of the thickness, the ferrite fraction is 60 to 90%, the effective grain size is 15 μm or less, and the coarse grain ratio, which is the area ratio of grains with a grain size of 20 μm or more, is 20%. It is below. When the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed with the RD surface is 45 °, and the angle with the TD surface is 45 °. In a specific plane having an angle of 45°, the degree of integration of {100} planes is 1.50 to 2.50.
0.300≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.380 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

Steel materials for line pipes are, for example, hot-rolled steel sheets for line pipes or electric resistance welded steel pipes for line pipes.

In the present embodiment, the effective crystal grain size is defined as the position where the orientation difference of adjacent measurement points exceeds 15 ° in the EBSP-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method as a grain boundary. , mean the area-average grain size calculated from the grain size and area obtained as crystal grains in the region surrounded by the grain boundaries.

The steel material for line pipes according to the present invention has excellent low temperature toughness.

Fig. 1 shows that, in steel material for line pipes, when the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed by the RD surface is 45. is 45° and forms an angle of 45° with the TD plane (hereinafter also referred to as a specific plane). ND indicates the thickness direction. FIG. 2 is a diagram showing the relationship between the degree of {100} plane accumulation ({100} degree of accumulation) in a specific plane and the ductile fracture surface ratio (%) when a DWTT test is performed at -30°C. FIG. 3 is a schematic diagram of a continuous cooling transformation curve of the steel material for line pipe according to the present embodiment. FIG. 4 is a flow chart showing the manufacturing process of the steel material for line pipes according to the present embodiment. FIG. 5 is a plan view of a tensile test piece used in the tensile test. In addition, the numerical value in a figure shows a dimension (unit: mm). FIG. 6 is a front view and a side view of a DWTT test piece used in the DWTT test. Numerical values and t in the figure indicate dimensions (unit: mm).

The present inventors have investigated and studied the low-temperature toughness of steel materials for line pipes, and obtained the following knowledge.

The present inventors first focused on the texture of steel in order to improve the low-temperature toughness of electric resistance welded steel pipes for line pipes.

Carbon steel, such as steel for line pipes, exhibits a body-centered cubic structure. A plane is a crystal plane that is likely to separate when a crack propagates parallel to the {100} plane.

Line pipes are filled with gas or liquid such as natural gas or crude oil at high pressure, and since the maximum stress acts in the circumferential direction of the steel pipe, cracks tend to propagate parallel to the steel pipe axis. In electric resistance welded steel pipes for line pipes, the pipe axis direction is usually the rolling direction (hereinafter referred to as the RD direction) during hot rolling of the hot-rolled steel plate for line pipes, so cracks tend to propagate in the RD direction. is.

Therefore, the inventors considered that cracks propagate easily when the normal to the {100} plane is perpendicular to the RD direction. Therefore, the present inventors considered that if the normal to the {100} plane is inclined by 45° from the direction perpendicular to the RD direction, crack propagation can be suppressed. This is because the normal to the {100} plane can be most inclined from the direction perpendicular to the RD direction. More specifically, it is as follows.

FIG. 1 is a schematic diagram of a minute area at the center of the thickness of a steel material for line pipes. When the steel material for line pipes is a steel plate, the minute region shown is the central portion of the plate thickness. When the steel material for line pipes is a steel pipe, the plate thickness central portion corresponds to the wall thickness central portion. In other words, the minute area shown in the drawing is the central part of the thickness. In FIG. 1, the plane perpendicular to the rolling direction is the RD plane, the rolling plane is the ND plane, and the RD plane and the plane perpendicular to the ND plane are the TD planes. A plane forming an angle of 45° with the RD plane and forming an angle of 45° with the TD plane is defined as a specific plane. The ND direction indicates the thickness direction (thickness direction or thickness direction).

Note that the circumferential direction of the steel pipe is curved, unlike the width direction of the steel plate. However, in the minute regions described above, the circumferential direction is substantially straight and substantially coincides with the straight TD direction. Therefore, the specific surface shown in FIG. 1 is similarly shown in the electric resistance welded steel pipe for line pipe.

Referring to FIG. 1, when the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed with the RD surface is 45°. And the inventors of the present application thought that if {100} planes were accumulated in a specific plane having an angle of 45° with the TD plane, peeling would be suppressed on the plane where cracks are most likely to grow. .

Based on the above ideas, the present inventors have determined the degree of integration of the {100} plane in a specific plane (hereinafter referred to as the {100} degree of integration) and the ductile fracture surface rate when performing a DWTT test at -30 ° C. (%).

FIG. 2 is a diagram showing the relationship between the {100} accumulation degree and the ductile fracture surface ratio (%) when the DWTT test is performed at -30°C. Figure 2 was obtained as follows.

Electric resistance welded steel pipes for line pipes were manufactured using slabs having the composition of Steel A shown in Table 1 in Examples described later.

Specifically, the slab was heated to a temperature of 1060-1200° C. in a heating furnace. Rough rolling was performed on the heated slab. The temperature T ₀ at the exit side of the final stand in rough rolling was 900 to 985° C., and the time t ₀ from immediately after the end of rough rolling to the start of finish rolling was 50 to 230 seconds. After rough rolling, finish rolling was performed at 760 to 800° C. to produce a steel plate. ROT (run out table) cooling was performed on the steel sheet after the finish rolling.

A steel plate was manufactured by the manufacturing process described above. The obtained hot-rolled steel sheet for line pipe was coiled to obtain a hot-rolled steel sheet for line pipe in the form of a hot coil.

Using the hot-rolled steel sheets for line pipes thus obtained, pipes were produced by the method described above to produce electric resistance welded steel pipes for line pipes having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 to 25 mm.

The {100} integration degree was measured using an EBSD (Electron Back Scatter Diffraction Patterns) method based on the method described below. The measurement conditions for the EBSD method were magnification: 400 times, visual field area: 200 μm×500 μm, and measurement step: 0.3 μm.

A low-temperature toughness test was performed on the obtained electric resistance welded steel pipe for line pipe. More specifically, a DWTT test was performed at −30° C. in accordance with API standard 5L3 (Recommended practice for conducting drop weight tear test) to determine the ductile fracture rate.

Referring to FIG. 2, as the {100} accumulation degree increases, the ductile fracture surface ratio increases. When the {100} accumulation degree exceeds 1.50, the ductile fracture surface ratio becomes 85% or more. When the {100} accumulation degree exceeds 1.50, the ductile fracture surface ratio does not change so much even when the {100} accumulation degree increases. That is, the ductile fracture surface ratio with respect to the {100} accumulation degree has an inflection point near the {100} accumulation degree of 1.50. As described above, the ductile fracture surface ratio becomes 85% or more at the inflection point near the {100} accumulation degree of 1.50, and the electric resistance welded steel pipe for line pipe having excellent low temperature toughness can be obtained. they found out for the first time.

The present inventors have further investigated and examined the low-temperature toughness of steel materials for line pipes, and obtained the following findings.

(A) If the structure of the steel is mainly ferrite, fine crystal grains can be obtained and the low-temperature toughness of the steel increases. Coarse grains are suppressed by setting the heating temperature during rolling to 1200° C. or less. Furthermore, cooling is controlled so that a large number of nucleation sites are generated in the non-recrystallized structure after rolling, and a large number of new ferrite grains are generated. In this case, the final ferrite grains become finer, resulting in an increase in the low temperature toughness of the steel.

On the other hand, if the structure of the steel is mainly bainite, although laths (elongated structures) are generated in the crystal grains inherited from the previous austenite grains, the orientations of the laths are aligned for each block, and each block is substantially the same. It becomes one crystal grain. Therefore, the size of crystal grains in bainite is determined by the size of prior austenite grains. Therefore, crystal grains tend to coarsen, and as a result, the low-temperature toughness of steel tends to decrease. Therefore, in the steel material for line pipes of the present embodiment, the structure is mainly ferrite.

(B) C, Si, Mn, Ni, Cr, Mo, V, and Nb are all S curves (ferrite region, pearlite area and bainite area).

Define F1=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3. If F1 is too low, the S-curve of the CCT diagram will shift too far to the left. In this case, the low temperature toughness of the steel is reduced. The reason for this is as follows.

The magnitude of the driving force (driving force for phase transformation) for transforming from austenite to ferrite correlates with the temperature of the steel material. When the steel material temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are less likely to be generated. Furthermore, since the temperature of the steel material is high, the growth of ferrite grains is rapid. As a result, ferrite grains become coarse. On the other hand, when the steel material temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. Furthermore, due to the low steel material temperature, the growth of ferrite grains is slow. As a result, ferrite grains are refined.

When the S curve of the CCT diagram shifts too far to the left, it enters the ferrite region while the steel material temperature is high. Therefore, as described above, the ferrite grains are coarsened and the effective crystal grain size is increased. Furthermore, since it tends to form a mixed grain structure, the ratio of coarse grains increases. In this case, the low temperature toughness of the steel is reduced. If F1 is too low, the hardenability is further lowered and sufficient strength cannot be obtained.

On the other hand, if F1 is too high, the S-curve will shift too far to the right. In this case, the cooling curve is less likely to overlap the ferrite nose. As a result, the amount of hard structures such as bainite and martensite generated increases, and the ferrite fraction in the structure decreases. As a result, the low temperature toughness of the steel is reduced.

FIG. 3 is an example of a Continuous Cooling Transformation Diagram (CCT diagram) of the steel material for line pipe according to the present embodiment. In FIG. 3, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose. When F1 is between 0.300 and 0.380, the S-curves of each phase (ferrite, pearlite, bainite) are placed at appropriate positions on the CCT diagram. In this case, it is possible to cool mainly through the ferrite region as shown by the cooling curve C1 in FIG. Therefore, a ferrite-based structure can be produced, and high strength and low-temperature toughness can be obtained.

The steel material for line pipes according to the present embodiment completed based on the above knowledge has C: 0.010 to 0.060%, Si: 0.05 to 0.30%, Mn: 0.50 to 0.50% by mass. 2.00%, P: 0-0.030%, S: 0-0.0100%, Al: 0.010-0.035%, N: 0.0010-0.0080%, Nb: 0.010 ~0.080%, Ti: 0.005-0.030%, Ni: 0.001-0.50%, Mo: 0.05-0.30%, O: 0-0.0030%, Ca: 0 to 0.0050%, V: 0 to 0.100%, Cr: 0 to 0.30%, Cu: 0 to 0.30%, Mg: 0 to 0.0050%, and rare earth elements: 0 to It contains 0.0100%, the balance being Fe and impurities, and has a chemical composition that satisfies formula (1). In the structure at the center of the thickness, the ferrite fraction is 60 to 90%, the effective grain size is 15 μm or less, and the coarse grain ratio, which is the area ratio of grains with a grain size of 20 μm or more, is 20%. It is below. When the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed with the RD surface is 45 °, and the angle with the TD surface is 45 °. In a specific plane having an angle of 45°, the degree of integration of {100} planes is 1.50 to 2.50.
0.300≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.380 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1).

The above chemical composition may contain Ca: greater than 0 to 0.0050%. The chemical composition is one or more selected from the group consisting of V: more than 0 to 0.100%, Cr: more than 0 to 0.30%, and Cu: more than 0 to 0.30%. may contain. The chemical composition may contain one or more selected from the group consisting of Mg: more than 0 to 0.0050% and rare earth elements: more than 0 to 0.0100%.

The steel materials for line pipes are, for example, hot-rolled steel sheets for line pipes and electric resistance welded steel pipes for line pipes.

The electric resistance welded steel pipe for line pipe has a yield strength of 450 to 540 MPa in the pipe axial direction and a tensile strength in the pipe axial direction of 510 to 625 MPa.

The electric resistance welded steel pipe for line pipe may have a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

Hereinafter, the steel material for line pipes of this embodiment will be described in detail. "%" for elements means % by weight unless otherwise specified.

[Chemical composition]
The steel material for line pipes of this embodiment is a hot-rolled steel plate for line pipes or an electric resistance welded steel pipe for line pipes. The chemical composition of steel for line pipes contains the following elements.

C: 0.010-0.060%
Carbon (C) increases the strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, carbides are formed and the low temperature toughness and ductility of the steel are reduced. Too high a C content further reduces weldability. Therefore, the C content is 0.010-0.060%. A preferred lower limit for the C content is 0.025%, more preferably 0.030%. A preferable upper limit of the C content is 0.058%.

Si: 0.05-0.30%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the low temperature toughness of the steel will decrease. Therefore, the Si content is 0.05-0.30%. A preferable lower limit of the Si content is 0.10%, more preferably 0.15%. A preferred upper limit for the Si content is 0.25%, more preferably 0.21%.

Mn: 0.50-2.00%
Manganese (Mn) increases the hardenability of steel and increases the strength of steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the strength of the steel will be too high and the low temperature toughness of the steel will be reduced. Therefore, the Mn content is 0.50-2.00%. A preferred lower limit for the Mn content is 0.80%, more preferably 1.00%. A preferable upper limit of the Mn content is 1.80%, more preferably 1.50%.

P: 0-0.030%
Phosphorus (P) is an impurity. P lowers the low temperature toughness of steel. Therefore, it is preferable that the P content is as low as possible. Specifically, the P content is 0 to 0.030%. A preferable upper limit of the P content is 0.020%, more preferably 0.015%. On the other hand, the P content may be 0%. However, from the viewpoint of dephosphorization cost reduction, the P content may be more than 0%, may be 0.001% or more, or may be 0.005% or more.

S: 0-0.0100%
Sulfur (S) is an impurity. S combines with Mn to form Mn-based sulfides. Therefore, the low temperature toughness and SSC resistance of the steel are lowered. Therefore, it is preferable that the S content is as low as possible. Specifically, the S content is 0 to 0.0100%. A preferable upper limit of the S content is 0.0080%, more preferably 0.0050%. On the other hand, the S content may be 0%. However, from the viewpoint of desulfurization cost reduction, the S content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0020% or more. There may be.

Al: 0.010-0.035%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, Al nitrides will coarsen and the low temperature toughness of the steel will deteriorate. Therefore, the Al content is 0.010-0.035%. A preferable lower limit of the Al content is 0.015%, more preferably 0.020%. A preferable upper limit of the Al content is 0.030%. As used herein, Al content means the total Al content in the steel.

N: 0.0010 to 0.0080%
Nitrogen (N) forms nitrides to suppress coarsening of austenite grains during the heating process. In this case, the austenite grains are refined in the rolling process, and the crystal grains after transformation are refined. As a result, the low temperature toughness of the steel is enhanced. N further increases the strength of steel through solid-solution strengthening. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, the carbonitrides are coarsened and the low temperature toughness of the steel is lowered. Therefore, the N content is 0.0010-0.0080%. A preferable lower limit of the N content is 0.0020%, more preferably 0.0025%. A preferable upper limit of the N content is 0.0060%, more preferably 0.0050%.

Nb: 0.010-0.080%
Niobium (Nb) combines with C and N in steel to form fine Nb carbonitrides. The Nb carbonitride suppresses grain coarsening and reduces the effective grain size. Therefore, it increases the low temperature toughness of the steel. Furthermore, fine Nb carbonitrides increase the strength of steel through dispersion strengthening. If the Nb content is too low, these effects cannot be obtained. On the other hand, if the Nb content is too high, the Nb carbonitrides become coarse and the low temperature toughness of the steel deteriorates. Therefore, the Nb content is 0.010-0.080%. A preferred lower limit for the Nb content is 0.015%. A preferable upper limit of the Nb content is 0.040%, more preferably 0.030%.

Ti: 0.005-0.030%
Titanium (Ti) combines with N in the steel to form TiN, and suppresses deterioration of the low temperature toughness of the steel due to dissolved N. Further, fine TiN is dispersed and precipitated, thereby suppressing coarsening of crystal grains. This increases the low temperature toughness of the steel. If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, TiN is coarsened or coarse TiC is produced. In this case, the low temperature toughness of the steel is reduced. Therefore, the Ti content is 0.005-0.030%. A preferable lower limit of the Ti content is 0.007%, more preferably 0.010%. A preferable upper limit of the Ti content is 0.020%, more preferably 0.017%.

Ni: 0.001-0.50%
Nickel (Ni) enhances the hardenability of steel and enhances the strength of steel. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Ni content is too high, this effect is saturated. Therefore, the Ni content is 0.001-0.50%. A preferable lower limit of the Ni content is 0.05%, more preferably 0.07%. A preferable upper limit of the Ni content is 0.20%.

Mo: 0.05-0.30%
Molybdenum (Mo) increases the hardenability of steel and increases the strength of steel. Mo further refines austenite grains and increases the low temperature toughness of steel. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the field weldability of the steel deteriorates. Therefore, the Mo content is 0.05-0.30%. A preferred lower limit for the Mo content is 0.15%. A preferred upper limit for the Mo content is 0.20%, more preferably 0.18%.

O: 0 to 0.0030%
Oxygen (O) is an impurity. O forms oxides and lowers the resistance to hydrogen-induced cracking of steel. O further reduces the low temperature toughness of steel. Therefore, it is preferable that the O content is as low as possible. Specifically, the O content is 0 to 0.0030%. A preferable upper limit of the O content is 0.0025%. On the other hand, the O content may be 0%. However, from the viewpoint of reducing the deoxidation cost, the O content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, or may be 0.0015% or more. or 0.0020% or more.

The remainder of the chemical composition of the steel material for line pipes according to the present embodiment consists of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material for line pipes is industrially manufactured, and have an adverse effect on the steel material for line pipes of the present embodiment. It means what is allowed as long as it does not give

[Regarding arbitrary elements]
The chemical composition of the steel material for line pipe described above may further contain Ca instead of part of Fe.

Ca: 0 to 0.0050%,
Calcium (Ca) is an optional element and may not be contained. When included, Ca controls the morphology of MnS to spheroidize. In this case, the low temperature toughness of the steel is enhanced. However, if the Ca content is too high, coarse oxide inclusions are formed. Therefore, the Ca content is 0-0.0050%. The Ca content may be 0%. A preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0010%, still more preferably 0.0015%. A preferred upper limit for the Ca content is 0.0045%.

The chemical composition of the steel material for line pipe described above may further contain one or more selected from the group consisting of V, Cr and Cu in place of part of Fe. These elements increase the strength of steel.

V: 0-0.100%
Vanadium (V) is an optional element and may not be contained. When contained, V combines with C and N in the steel during the winding process to form fine carbonitrides and increases the strength of the steel. Fine V carbonitrides also suppress coarsening of grains and enhance the low temperature toughness of steel. However, if the V content is too high, V carbonitrides coarsen and the low temperature toughness of the steel decreases. Therefore, the V content is 0-0.100%. The V content may be 0%. A preferable lower limit of the V content is more than 0%, more preferably 0.001%, and still more preferably 0.002%. A preferable upper limit of the V content is 0.080%, more preferably 0.060%.

Cr: 0-0.30%
Chromium (Cr) is an optional element and may not be contained. When included, Cr increases the hardenability of the steel and increases the strength of the steel. However, if the Cr content is too high, the hardenability will be too high and the low temperature toughness of the steel will be reduced. Therefore, the Cr content is 0-0.30%. The Cr content may be 0%. A preferable lower limit of the Cr content is over 0%, more preferably 0.01%. A preferable upper limit of the Cr content is 0.20%, more preferably 0.10%, and still more preferably 0.05%.

Cu: 0-0.30%
Copper (Cu) is an optional element and may not be contained. When included, Cu enhances the hardenability of the steel and enhances the strength of the steel. However, if the Cu content is too high, the hardenability will be too high and the low temperature toughness of the steel will be reduced. Too high a Cu content also causes surface cracking during casting and hot rolling due to liquid metal embrittlement. Therefore, the Cu content is 0-0.30%. The Cu content may be 0%. A preferable lower limit of the Cu content is more than 0%, more preferably 0.01%, more preferably 0.05%, still more preferably 0.10%. A preferable upper limit of the Cu content is 0.25%, more preferably 0.20%.

The chemical composition of the steel material for line pipe described above may further contain one or more selected from the group consisting of Mg and rare earth elements instead of part of Fe. These elements function as deoxidizers and desulfurizers.

Mg: 0-0.0050%
Magnesium (Mg) is an optional element and may not be contained. When included, Mg functions as a deoxidizing and desulfurizing agent. In addition, it produces fine oxides and contributes to the improvement of HAZ toughness. However, if the Mg content is too high, the oxide tends to aggregate or coarsen. As a result, the HIC resistance may be lowered, or the toughness of the base metal portion or HAZ may be lowered. Therefore, the Mg content is 0-0.0050%. The Mg content may be 0%. A preferable lower limit of the Mg content is more than 0%, more preferably 0.0001%, and still more preferably 0.0010%. A preferred upper limit for the Mg content is 0.0030%.

Rare earth elements: 0 to 0.0100%
Rare earth elements (REM) are optional elements and need not be included. When included, REM functions as a deoxidizing and desulfurizing agent. However, if the REM content is too high, coarse oxides are produced. As a result, the HIC resistance may be lowered, or the toughness of the base metal portion or HAZ may be lowered. Therefore, the REM content is 0-0.0100%. The REM content may be 0%. A preferable lower limit of the REM content is more than 0%, more preferably 0.0001%, and more preferably 0.0010%. A preferred upper limit for the REM content is 0.0070%, more preferably 0.0050%.

Here, REM is yttrium (Y) with an atomic number of 39, lanthanide (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, which is a lanthanoid, and actinium with an atomic number of 89, which is an actinide. It is one or more elements selected from the group consisting of (Ac) to No. 103 lawrencium (Lr).

[Regarding formula (1)]
The above chemical composition further satisfies formula (1).
0.300≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.380 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1). Further, when the element corresponding to the element symbol in formula (1) is not contained, "0" is substituted for the corresponding element symbol in formula (1).

As described above, in the chemical composition of this embodiment, the contents of C, Si, Mn, Ni, Cr, Mo, V, and Nb improve the hardenability of the steel.

Define F1=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3. If F1 is too low, the S-curve of the CCT diagram will shift too far to the left. In this case, the low temperature toughness of the steel is reduced. The reason for this is as follows.

The magnitude of the driving force (driving force for phase transformation) for transforming from austenite to ferrite correlates with the temperature of the steel material. When the steel material temperature is high, the driving force for phase transformation is small. Therefore, ferrite transformation nuclei are less likely to be generated. However, since the steel material temperature is high, the growth of ferrite grains is rapid. As a result, ferrite grains become coarse. On the other hand, when the steel material temperature is low, the driving force for phase transformation is large. Therefore, ferrite transformation nuclei are likely to be generated. However, since the steel material temperature is low, the growth of ferrite grains is slow. As a result, ferrite grains are refined.

When the S curve of the CCT diagram shifts too far to the left, the steel material temperature enters the ferrite region in a high state. Therefore, as described above, the ferrite grains are coarsened and the effective crystal grain size is increased. Furthermore, since it tends to form a mixed grain structure, the ratio of coarse grains increases. In this case, the low temperature toughness of the steel is reduced. If F1 is too low, the hardenability is further lowered and sufficient strength cannot be obtained.

On the other hand, if F1 is too high, the S curve of the CCT diagram shifts to the right (longer time side). In this case, hard structures such as bainite and martensite are likely to form, and the ferrite fraction in the structure decreases. As a result, the low temperature toughness of the steel is reduced. If F1 is too high, the hardenability of the steel will be too high and the low temperature toughness of the steel will be reduced.

When F1 is between 0.300 and 0.380, the temperature of the steel is easily maintained in the ferrite region when the cooling curve C1 shown in FIG. 3 is performed. Therefore, the ferrite fraction in the thickness central portion of the steel material can be 60 to 90%, and the low-temperature toughness of the steel can be enhanced. A preferred upper limit for F1 is 0.35.

[Regarding the ferrite fraction]
The structure of the central portion of the thickness of the steel material for line pipe according to the present embodiment is composed of ferrite, bainitic ferrite, and pearlite, and the remainder is precipitates and/or inclusions. Here, the thickness center part is a range of ± 20% t in the plate thickness direction or the thickness direction from the plate thickness center or the thickness center when the plate thickness or wall thickness is t mm (that is, from the surface to the plate 40 to 60% t in the thickness direction or in the thickness direction).

As described above, if the ferrite fraction in the structure at the center of the thickness of the steel is 60% or more and the ferrite grains are fine, the low-temperature toughness of the steel increases. In this specification, the ferrite fraction means the area ratio of ferrite.

If the ferrite fraction is less than 60%, the effective grain size and/or coarse grain ratio become too large. As a result, the low temperature toughness is lowered.

On the other hand, in the chemical composition of the present embodiment containing C, a metal structure having a ferrite fraction of 90% or less is likely to be formed.

Therefore, the ferrite fraction is 60 to 90% in the structure of the central part of the thickness of the steel material for line pipe according to the present embodiment. A preferable lower limit of the ferrite fraction is 65%, more preferably 70%, and still more preferably 75%.

A ferrite fraction means a ferrite area ratio and is measured by the following method. A sample is taken from the center of the thickness of the line pipe steel. When the steel material for line pipes is an electric resistance welded steel pipe for line pipes, a sample is taken from the thickness central portion at a position shifted by 90° in the pipe circumferential direction from the electric resistance welded portion.

The sample taken is polished with a colloidal silica abrasive for 30-60 minutes. The polished sample is analyzed using EBSP-OIM™ to determine the ferrite fraction. The visual field range is 200 μm (rolling direction)×500 μm (thickness direction) centered on the center of the thickness. The observation magnification is 400 times and the measurement step is 0.3 μm.

Specifically, the ferrite fraction is obtained by the KAM (Kernel Average Misorientation) method provided in EBSP-OIM (trademark).

In the KAM method, any one regular hexagonal pixel in the measurement data is taken as the central pixel. A first approximation using the 6 pixels adjacent to this center pixel (7 pixels in total), or a second approximation using 12 pixels outside of these 6 pixels (19 pixels in total). , or a third approximation using 18 pixels outside of these 12 pixels (total of 37 pixels), obtain the orientation difference between each pixel. The obtained orientation differences are averaged, and the obtained average value is taken as the value of the central pixel. This operation is performed for all pixels. In the present embodiment, pixels with an orientation difference of 5° or less between adjacent pixels by the third approximation are displayed. In the present embodiment, the ferrite fraction is defined as the area fraction of pixels calculated to have a misorientation of 1° or less in the third approximation with respect to the entire area of the visual field range. A structure other than ferrite such as bainite has a misorientation exceeding 1° in the third approximation.

[Effective grain size]
Further, in the steel material for line pipes of the present embodiment, the effective grain size at the central portion of the thickness of the steel material for line pipes is 15 μm or less. If the effective grain size is too large, the low temperature toughness of the steel will decrease. In the present embodiment, since the effective grain size is 15 μm or less, excellent low temperature toughness can be obtained. A preferable upper limit of the effective crystal grain size is 13 μm, more preferably 10 μm.

Effective grain size is measured using EBSP-OIM™. Specifically, a sample is taken and polished in the same manner as the measurement of the ferrite fraction. Polished samples are analyzed using EBSP-OIM™. More specifically, the grain boundary is defined as a position where the orientation difference between adjacent measurement points exceeds 15° in the orientation measurement at each fixed measurement step. 15° is the threshold value of the large tilt angle grain boundary, which is generally recognized as the grain boundary. A region surrounded by grain boundaries is defined as a crystal grain, and the grain size and the area of the crystal grain are obtained. The area-average particle size is obtained from the obtained particle size and area . In this specification, the determined area-average grain size is defined as the effective crystal grain size. The visual field range is 200 μm (rolling direction)×500 μm (thickness direction) centering on the center of the thickness. The observation magnification is 400 times and the measurement step is 0.3 μm.

[Regarding coarse grain ratio]
In the measurement using the above-mentioned EBSP-OIM (trademark), the area ratio of crystal grains having a grain size of 20 μm or more at the center of the thickness of the steel material for line pipe is defined as the “coarse grain ratio”. If the grains are coarse, the low temperature toughness of the steel is reduced. If the coarse grain ratio is 20% or less, excellent low temperature toughness can be obtained. A preferable upper limit of the coarse grain ratio is 18%, more preferably 15%. The lower the coarse grain ratio, the better.

Coarse grain ratio is measured using EBSP-OIM™. Samples are taken and polished as in the ferrite fraction measurement. Polished samples are analyzed using EBSP-OIM™. The visual field range is 200 μm (rolling direction)×500 μm (thickness direction) centered on the center of the thickness. The observation magnification is 400 times and the measurement step is 0.3 μm. It can be obtained by substituting N for the area of the measurement object observed in the EBSP-OIM (trademark) measurement and n for the area of the coarse crystal grains into the formula (2).
Coarse grain ratio (%) = (n/N) x 100 (2)

[The degree of integration of the {100} plane in the specific plane: 1.50 to 2.50]
In the steel material for line pipe according to the present embodiment, when the surface perpendicular to the rolling direction is the RD surface, the rolled surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed with the RD surface is 45. ° and the angle formed with the TD plane is 45°, the degree of integration of the {100} plane is 1.50 to 2.50. This suppresses delamination on the surface where cracks are most likely to grow. As a result, propagation of cracks can be suppressed and the low temperature toughness of the steel is enhanced.

When the {100} density is less than 1.50, the low temperature toughness is lowered. Although the upper limit of the {100} integration degree is not particularly limited, 2.50 at the DWTT guaranteed temperature of -30°C is sufficient. Therefore, the {100} density is 1.50-2.50. A preferred lower limit of the {100} density is 1.60, more preferably 1.70, still more preferably 1.80, and still more preferably 2.00. A preferred upper limit for the {100} density is 2.40.

[Method for measuring {100} degree of integration]
{100} density is measured using EBSP-OIM™. Specifically, a sample is taken from the center of the thickness of the line pipe steel material. When the steel material for line pipes is an electric resistance welded steel pipe for line pipes, a sample is taken from the thickness central portion at a position shifted by 90° in the pipe circumferential direction from the electric resistance welded portion.

The sample taken is polished with a colloidal silica abrasive for 30-60 minutes. Polished samples are analyzed using the EBSP-OIM™ EBSD method. The measurement conditions for the EBSD method are magnification: 400 times, visual field area: 200 μm×500 μm, and measurement step: 0.3 μm. By EBSD measurement, using the spherical harmonics method, the {100} concentration is obtained by texture analysis of the inverse pole figure in the direction perpendicular to the specific plane.

Microscopically, the circumferential direction of the electric resistance welded steel pipe for line pipes coincides with the TD direction of the hot-rolled steel plate for line pipes. Therefore, the specific surface shown in FIG. 1 is similarly shown in the electric resistance welded steel pipe for line pipe. Therefore, regardless of whether the steel material for line pipes is an electric resistance welded steel pipe for line pipes or a hot-rolled steel plate for line pipes, the {100} accumulation degree is similarly measured.

By carrying out the manufacturing process described below, the ferrite fraction can be 60 to 90% or more, the effective grain size can be 15 μm or less, and the coarse grain ratio can be 20% or less in the structure at the center of the thickness. . Furthermore, the {100} density can be 1.50 to 2.50. As a result, the DWTT guarantee temperature can be set to −30° C. or lower, and the low temperature toughness can be enhanced.

[Yield strength YS in the tube axis direction]
When the steel material for line pipes of this embodiment is an electric resistance welded steel pipe for line pipes, the yield strength YS in the pipe axial direction is preferably 450 to 540 MPa. If the yield strength YS is 450 MPa or more, the strength required for electric resistance welded steel pipes for line pipes can be easily satisfied. If the yield strength YS is 540 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline formed using the electric resistance welded steel pipe for line pipe. A more preferable lower limit of the yield strength YS is 460 MPa, more preferably 480 MPa. A more preferable upper limit of the yield strength YS is 530 MPa, more preferably 520 MPa.

[Tensile strength TS in the tube axis direction]
When the steel material for line pipes of this embodiment is an electric resistance welded steel pipe for line pipes, the tensile strength TS in the pipe axial direction is preferably 510 to 625 MPa. If the tensile strength TS is 510 MPa or more, the strength required for electric resistance welded steel pipes for line pipes can be easily satisfied. If the tensile strength TS is 625 MPa or less, it is advantageous in suppressing bending deformation or buckling when laying a pipeline formed using the electric resistance welded steel pipe for line pipe. A more preferable lower limit of the tensile strength TS is 530 MPa, more preferably 540 MPa, still more preferably 545 MPa. A more preferable upper limit of the tensile strength TS is 620 MPa, more preferably 600 MPa.

Yield strength YS and tensile strength TS can be measured by the following methods. A full-thickness tensile test piece is taken from a position shifted by 90° in the pipe circumferential direction from the electric resistance welded portion of the electric resistance welded steel pipe for line pipe. The longitudinal direction of the tensile test piece is parallel to the axial direction of the electric resistance welded steel pipe for line pipe. The cross section of the tensile test piece (the cross section parallel to the width direction and thickness direction of the tensile test piece) has an arc shape. The parallel portion of the tensile test piece has a length of 50.8 mm and a width of 38.1 mm. In the present embodiment, the above tensile test piece is used to perform a tensile test at room temperature in accordance with the API standard 5CT. Normal temperature is, for example, 24°C. Yield strength YS and tensile strength TS are determined based on the results of the tensile test.

[Production method]
An example of a method for manufacturing the steel material for line pipes will be described. FIG. 4 is a flow diagram showing an example of a manufacturing process of steel material for line pipes.

Referring to FIG. 4, in this manufacturing method, a slab as a material is manufactured using molten steel satisfying the chemical composition described above (material preparation step: S0). The manufactured slab is heated in a heating furnace (heating step: S1). The heated slab is rolled by a rough rolling mill and a finishing rolling mill to produce a steel plate (rolling step: S2). In the rolling step (S2), the slab is subjected to rough rolling to produce a rough rolled plate (rough rolling step: S21). The rough rolled plate is subjected to finish rolling by a finish rolling mill to produce a steel plate (finish rolling step: S22). The manufactured steel plate is cooled by ROT (run-out table) (ROT cooling step: S3). In the ROT cooling step (S3), first, the steel sheet is strongly cooled by a water cooling device (strong cooling step S31). After intense cooling, the steel plate is slowly cooled (slow cooling step: S32). The steel sheet after ROT cooling is wound up (winding step: S4). A hot-rolled steel sheet for line pipes is manufactured through the above-described manufacturing process.

Further, the hot-rolled steel sheet for line pipe is shaped and welded to produce a pipe, thereby producing an electric resistance welded steel pipe for line pipe (pipe making step: S5). Each step will be described in detail below.

[Material preparation step (S0)]
A material having the chemical composition described above is prepared. Specifically, molten steel having the chemical composition described above is produced. A raw material (slab) is manufactured using molten steel. A cast piece (slab) may be produced by a continuous casting method. An ingot may be manufactured using molten steel, and the ingot may be bloomed to manufacture a raw material (slab).

[Heating step (S1)]
In the heating step (S1), the manufactured slab is heated in a heating furnace. The heating temperature of the slab in the heating furnace is preferably 1060-1200°C. If the heating temperature is 1060° C. or higher, precipitation strengthening after rolling can be obtained, and appropriate strength can be obtained. If the heating temperature is 1200° C. or less, coarsening of crystal grains (austenite grains) can be suppressed. If the heating temperature is 1200° C. or less, the temperature T ₀ at the exit side of the final stand in the next step of rough rolling can be maintained appropriately. Therefore, the heating temperature is 1060-1200°C. A preferable lower limit of the heating temperature is 1100°C. A preferable upper limit of the heating temperature is 1160°C.

[Rolling step (S2)]
In the rolling step (S2), the slab heated in the heating step (S1) is hot-rolled using a rough rolling mill and a finishing rolling mill to form a steel plate. The rolling process (S2) includes a rough rolling process (S21) and a finish rolling process (S22). Both the roughing mill and the finishing mill comprise a plurality of rolling stands arranged in a row, each rolling stand comprising a pair of rolls.

[Rough rolling step (S21)]
In the rough rolling step (S21), the prepared slab is subjected to rough rolling to produce a rough rolled sheet.

As the rough hot rolling mill, for example, a multistage hot rolling mill having a plurality of stands is used. For example, there is a method in which reciprocating or unidirectional rolling is performed using a two-high or four-high hot rolling mill with 1 to 3 stands.

The total rolling reduction in rough rolling is not particularly limited as long as the effects of the present embodiment can be obtained, but is preferably 60 to 75%.

The time from immediately after the end of rough rolling to the start of finish rolling in the next step is controlled according to the temperature at the delivery side of the final stand for rough rolling. Note that "immediately after completion of rough rolling" means within 5 m from the exit side of the final stand for rough rolling.

The shorter the time from immediately after the end of rough rolling to the start of finish rolling, the better. If the time from the end of rough rolling to the start of finish rolling is short, it becomes difficult to recrystallize the steel after rough rolling and before finish rolling. In this case, the finish rolling can be performed while maintaining the shape of the crystal grains flattened by the rough rolling and the working strain introduced into the grains. As a result, in the structure after finish rolling and cooling, the refinement of the average crystal grain size and the accumulation of {100} planes on specific planes are further facilitated.

The time from immediately after the end of rough rolling to the start of finish rolling can be changed according to the temperature on the delivery side of the final stand for rough rolling. More specifically, if the temperature on the delivery side of the final stand in rough rolling is low, it becomes easier to maintain the shape of flattened crystal grains. In this case, the time from immediately after the end of rough rolling to the start of finish rolling can be lengthened. If the temperature on the delivery side of the final stand in rough rolling is high, it becomes difficult to maintain the shape of the flattened crystal grains. In this case, it is necessary to shorten the time from immediately after the end of rough rolling to the start of finish rolling.

The temperature T ₀ (° C.) at the delivery side of the final stand for rough rolling and the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling satisfy the following equation (3).
t ₀ (s)≦−3.7T ₀ +3686 (3)
Define F2=-3.7T ₀ +3686. When the heating temperature is within the above range and t ₀ (s) is F2 or less, recrystallization does not occur and the shape of crystal grains flattened by rough rolling can be easily maintained. As a result, it becomes easier for the {100} planes to accumulate on the specific plane. On the other hand, if t ₀ (s) exceeds F2, recrystallization occurs and the shape of crystal grains flattened by rough rolling cannot be maintained. As a result, {100} planes are less likely to accumulate on a specific plane.

[Finish rolling step (S22)]
In the finish rolling step, the obtained rough rolled plate is subjected to finish rolling by a finish rolling mill to produce a steel plate.

In the finish rolling process, tandem rolling may be performed using a tandem rolling mill including a plurality of rolling stands arranged in a row (each rolling stand having a pair of work rolls) to perform a plurality of passes. , reverse rolling may be performed by a Sendzimir rolling mill or the like having a pair of work rolls, and a plurality of passes may be performed.

In the finish rolling process, the surface temperature of the steel sheet on the delivery side of the final stand of the finish rolling mill is defined as the finish rolling temperature (°C). The finish rolling temperature (°C) is preferably low. A low temperature is, specifically, 800° C. or lower. If the finish rolling temperature is 800° C. or less, the rolling texture and its transformation texture develop. Thereby, the {100} integration degree can be increased.

However, the finish rolling temperature (°C) is preferably equal to or higher than the Ar ₃ transformation temperature. If the finish rolling temperature is equal to or higher than the Ar ₃ transformation temperature, the rolling resistance of the steel sheet can be reduced, and productivity increases. If the finish rolling temperature is equal to or higher than the Ar ₃ transformation temperature, it is possible to further prevent the steel sheet from being rolled in the two-phase region of ferrite and austenite. In this case, it is possible to suppress the microstructure of the steel sheet from forming a layered structure, and the mechanical properties are enhanced. Therefore, the finish rolling temperature is preferably equal to or higher than the _Ar3 transformation temperature. The Ar ₃ transformation temperature of the line pipe steel material of the present embodiment having the chemical composition described above is 700 to 750°C.

The total rolling reduction in finish rolling is preferably 60 to 80%. In this case, the {100} integration degree is further increased.

From the above, when the time t ₀ (s) from immediately after the end of rough rolling to the start of finish rolling is F2 or less and the finish rolling temperature is low, the {100} accumulation degree is 1.50 or more.

The thickness of the steel sheet after finish rolling is 12 to 25 mm. By using the manufacturing method of the present embodiment, excellent toughness can be obtained even if the plate thickness is 12 mm or more.

[ROT cooling step (S3)]
In the ROT (run out table) cooling step (S3), the steel plate manufactured in the rolling step (S2) is cooled. The ROT cooling step (S3) preferably includes a strong cooling step (S31) and a slow cooling step (S32). As a result, the ferrite fraction increases in the structure at the center of the thickness of the steel material for line pipes, and the low-temperature toughness of the steel increases. This point will be described in detail below.

FIG. 3 is an example of a CCT diagram of the steel material for line pipe according to this embodiment. In FIG. 3, F indicates ferrite nose, P indicates pearlite nose, and B indicates bainite nose.

As shown in FIG. 3, the ferrite nose exists at a higher position than the pearlite nose and the bainite nose. A dashed line C2 in FIG. 3 indicates a cooling curve (cooling curve C2) according to a conventional cooling process. The cooling curve C2 may pass through the pearlite nose. Conventional cooling methods pass through all of the ferrite nose, pearlite nose and/or bainite nose at a uniform rate during the cooling process. Therefore, a large amount of pearlite and/or bainite is generated in the structure, and the ferrite fraction in the structure is reduced.

Therefore, in the present embodiment, cooling is performed, for example, along the cooling curve (cooling curve C1) indicated by the dashed line C1. Specifically, in the early stages of cooling, strong cooling is performed up to the vicinity of the ferrite nose (S31). When the steel is cooled rapidly by intense cooling, it creates multiple strains in the steel, resulting in multiple nucleation sites in the unrecrystallized structure. After strong cooling, slow cooling is performed (S32). At this time, the temperature of the steel is kept within the ferrite region in FIG. As a result, fine ferrite is generated from numerous nucleation sites generated during intense cooling. As a result, the ferrite fraction in the structure is increased and the crystal grains are refined. Therefore, the low temperature toughness of the steel is enhanced. Cooling curve C1 may pass through the pearlite nose.

[Strong cooling step (S31)]
First, the steel sheet is strongly cooled. Intensive cooling is, for example, water cooling with a water cooling device. Although the surface temperature of the steel sheet immediately before water cooling is not particularly limited, it is preferably the Ar ₃ transformation temperature or higher. If the surface temperature of the steel sheet immediately before water cooling is equal to or higher than the Ar ₃ transformation temperature, it is possible to prevent reduction in strength due to coarsening of crystal grains due to grain growth.

Let the cooling rate in the strong cooling step (S31) be V1 (°C/s). V1 is calculated by heat conduction. V1 is preferably 5° C./s or more at the plate thickness central portion. If the cooling rate V1 is less than 5° C./s, the degree of supercooling due to cooling is insufficient, and sufficient ferrite nucleation sites cannot be obtained. In this case, since the amount of ferrite grains produced is reduced, the ferrite grains are coarsened and the low-temperature toughness of the steel is lowered. Therefore, the cooling rate V1 is 5° C./s or more. A preferable lower limit of the cooling rate V1 is 7° C./s, more preferably 8° C./s.

In the strong cooling step (S31), the steel plate is cooled until the surface temperature of the steel plate reaches 580 to 680°C. In other words, the strong cooling stop temperature T1 is 580-680.degree. If the strong cooling stop temperature T1 is too low, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. On the other hand, if the strong cooling stop temperature T1 is too high, the precipitation of Nb, which strengthens the pro-eutectoid ferrite, is overaged, and the strength of the steel is lowered. If the strong cooling stop temperature T1 is set to 580 to 680° C., the slow cooling in the subsequent slow cooling step (S4) can increase the ferrite fraction to 60% or more, increasing the low temperature toughness of the steel. The strong cooling stop temperature T1 is preferably 600 to 670°C, more preferably 610 to 670°C.

[Slow cooling step (S32)]
Slow cooling is performed with respect to the steel plate hard-cooled by the strong-cooling process (S31).

Let the cooling rate in the slow cooling step (S32) be V2 (°C/s). The cooling rate V2 is preferably 2.0 to 4.0° C./s at the plate thickness central portion. If the cooling rate V2 is too slow, the slow cooling stop temperature T2 and the winding temperature T3 in the subsequent steps will be too high. In this case, the crystal grains become coarse and the low temperature toughness of the steel deteriorates. If the cooling rate V2 is too fast, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. Therefore, the cooling rate V2 is 2.0-4.0° C./s.

In the slow cooling step (S32), the steel plate is cooled until the surface temperature of the steel plate reaches 500 to 670°C. In other words, the slow cooling stop temperature T2 is 500-670.degree. If the slow cooling stop temperature T2 is too low, the steel sheet temperature passes through the ferrite region and reaches the pearlite region and/or the bainite region in the CCT diagram. In this case, the ferrite fraction decreases and the low temperature toughness of the steel decreases. If the slow cooling stop temperature T2 is too high, the strength of steel will decrease. Therefore, the slow cooling stop temperature T2 is 500 to 670°C. A preferable lower limit of the slow cooling stop temperature T2 is 580°C, more preferably 590°C. The upper limit of the slow cooling stop temperature T2 is preferably 650°C, more preferably 635°C, and still more preferably 620°C.

[Winding step (S4)]
In the coiling process (S4), the steel sheet cooled in the ROT cooling process (S3) is coiled into a coil-shaped hot-rolled steel sheet for line pipes.

After the slow cooling step, the coiled hot-rolled steel sheet for line pipe is air-cooled and then coiled. The surface temperature T3 of the steel sheet during winding (hereinafter referred to as winding temperature) is 500 to 650°C. If the coiling temperature T3 is too low, the coarse grain ratio increases and the low temperature toughness of the steel decreases. On the other hand, if the coiling temperature T3 is too high, the crystal grains become coarse and the low temperature toughness of the steel deteriorates. Therefore, the winding temperature T3 is 500-650.degree. T3 is preferably 510-600°C, more preferably 520-560°C.

The hot-rolled steel sheet for line pipes of the present embodiment is manufactured by the manufacturing process described above.

[Pipe manufacturing step (S5)]
An electric resistance welded steel pipe for line pipe is manufactured by a well-known method while unwinding the coiled hot-rolled steel plate for line pipe. Specifically, a hot-rolled steel sheet for line pipe is bent into a tubular shape (open pipe) by continuous forming rolls. Subsequently, the seams of the open pipe, that is, both longitudinal end surfaces of the hot-rolled steel sheet for line pipe are welded by the electric resistance welding method. Through the above steps, an electric resistance welded steel pipe for line pipes is manufactured.

In the steel materials for line pipes (hot-rolled steel sheets for line pipes and electric resistance welded steel pipes for line pipes) manufactured by the above manufacturing process, the ferrite fraction is 60 to 90% or more in the structure at the center of the thickness, and the effective grain size is The diameter can be 15 μm or less and the coarse grain ratio can be 20% or less. Furthermore, the {100} density can be 1.50 to 2.50. As a result, the DWTT guarantee temperature can be set to −30° C. or lower, and the low temperature toughness can be enhanced. Furthermore, it is possible to obtain a yield stress of 450 to 540 MPa and a tensile strength of 510 to 625 MPa in the axial direction of the electric resistance welded steel pipe for line pipe.

Slabs were manufactured by continuously casting molten steels of Steel A to Steel M shown in Table 1.

Using a plurality of slabs of Steel A to Steel M, electric resistance welded steel pipes for line pipes of Test Nos. 1 to 21 shown in Table 2 were manufactured.

Specifically, the slab was heated to the heating temperature shown in Table 2 in a heating furnace. Rough rolling was performed on the heated slab. Table 2 shows the temperature T ₀ (° C.) at the exit side of the final stand in rough rolling, the time t ₀ (seconds) from immediately after the end of rough rolling to the start of finish rolling, and F2. After rough rolling, finish rolling was performed at the finish rolling temperature shown in Table 2 to produce a steel plate. The rolling reduction in the non-recrystallization temperature range was 60 to 80% for all test numbers. The finish rolling temperature was higher than the Ar ₃ transformation temperature except for test No. 19.

ROT cooling was implemented with respect to the steel plate after finish rolling. The time from the end of finish rolling to the start of strong cooling was set within 20 seconds. In the ROT cooling process, except for Test No. 20, the cooling rate V1 was 5°C/s or more, and the cooling was intensively cooled to the intensive cooling stop temperature T1 of 580 to 680°C. Then, it was slowly cooled at a cooling rate V2 of 2.0 to 4.0° C./s until it reached a slow cooling stop temperature T2 of 500 to 670° C. (provided that T1>T2 was satisfied).

A steel plate was manufactured by the manufacturing process described above. The obtained steel sheet was coiled at a coiling temperature T3 of 500 to 650° C. (where T2>T3 is satisfied) to obtain a hot-rolled steel sheet for line pipe in the form of a hot coil.

Using the obtained hot-rolled steel sheet for line pipe, a pipe was produced by the method described above to produce an electric resistance welded steel pipe for line pipe having an outer diameter of 304.8 to 660.4 mm and a wall thickness of 12 mm or more.

[Test method]
[Strength test]
Tensile test pieces were taken from electric resistance welded steel pipes for line pipes of each test number. Specifically, from the position 90° from the welded portion of the electric resistance welded steel pipe for line pipe when viewed in the axial direction (position shifted by 90° in the pipe circumferential direction from the electric resistance welded steel pipe), the full thickness A tensile test piece in the direction of the tube axis was taken. The cross section of the tensile test piece was arc-shaped, and the longitudinal direction of the tensile test piece was parallel to the longitudinal direction of the steel pipe. The size of the tensile test piece was as shown in FIG. 5, the length of the parallel portion was 50.8 mm, and the width of the parallel portion was 38.1 mm. Numerical values in FIG. 5 indicate dimensions (unit: mm) of corresponding portions of the test piece. Using a tensile test piece, a tensile test was carried out at room temperature in accordance with API standard 5CT. Yield strength YS (MPa) and tensile strength TS (MPa) of electric resistance welded steel pipes for line pipes were obtained based on the test results.

[Microstructure]
Regarding the electric resistance welded steel pipe for line pipe, the ferrite fraction, effective grain size, and coarse grain ratio at the center of the thickness were measured using EBSP-OIM (trademark) based on the method described above. The measurement conditions of EBSP-OIM (trademark) in effective crystal grain size measurement were magnification: 400 times, visual field area: 200 μm×500 μm, measurement step: 0.3 μm.

As analysis software for EBSP-OIM (trademark), "TSL OIM Analysis 7 (trademark)" manufactured by TSL Solutions was used.

In addition, in the measurement of the ferrite fraction, the type of the remainder (that is, the structure other than ferrite) in the metal structure of the thickness central portion of the base material portion was also confirmed.

[{100} degree of integration]
Regarding the electric resistance welded steel pipe for line pipe, the {100} accumulation degree was measured using EBSP-OIM (trademark) based on the method described above. The measurement conditions for EBSP-OIM (trademark) were magnification: 400 times, visual field area: 200 μm×500 μm, and measurement step: 0.3 μm.

[Low temperature toughness test]
A DWTT test piece was taken from the electric resistance welded steel pipe for line pipe of each test number. The sampling position was 90° from the same weld as the tensile specimen. An arc-shaped member sampled in the pipe circumferential direction from the sampling position was developed into a flat plate, and a notch was processed at a 90° position. The size of the DWTT specimen was as shown in FIG. Numerical values in FIG. 6 indicate dimensions (unit: mm) of corresponding portions of the test piece. t indicates the thickness (unit: mm). The longitudinal direction of the DWTT test piece corresponded to the circumferential direction of the electric resistance welded steel pipe for line pipe. A DWTT test was performed on the DWTT specimens in accordance with the ASTM E 436 specification. The minimum temperature (DWTT guarantee temperature) at which the ductile fracture surface ratio is 85% was determined. When the DWTT guarantee temperature was −30° C. or lower, it was evaluated that the low temperature toughness was high.

[Test results]
Table 3 shows the test results. In Table 3, "P, B" means at least one of pearlite and bainite.

With reference to Tables 1 to 3, the chemical compositions of the steels of Test Nos. 1 to 13 were appropriate and satisfied formula (1). Furthermore, the manufacturing conditions for any test number were appropriate. Therefore, in Test Nos. 1 to 13, the ferrite fraction was 60 to 90%, the effective grain size was 15 μm or less, and the coarse grain ratio was 20% or less. Furthermore, the {100} density was 1.50-2.50. As a result, the DWTT guaranteed temperature was −30° C. or lower, indicating excellent low temperature toughness. Furthermore, the yield strength YS in the pipe axial direction of the electric resistance welded steel pipes for line pipes was 450 to 540 MPa, and the tensile strength TS was 510 to 625 MPa.

On the other hand, in Test No. 14, although the manufacturing conditions were appropriate, F1 was less than the lower limit of formula (1). Therefore, the crystal grains became coarse, and the effective crystal grain size exceeded 15 μm. Furthermore, the {100} density was less than 1.50. Therefore, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In Test No. 15, although the manufacturing conditions were appropriate, F1 exceeded the upper limit of formula (1). As a result, the ferrite fraction was less than 60%, resulting in a bainite-based structure. Because of the bainite-based structure, the {100} accumulation degree was 1.50 or more, but the effective grain size exceeded 15 μm and the coarse grain ratio exceeded 20%. Therefore, the DWTT guaranteed temperature was higher than -30°C and the low temperature toughness was low. Furthermore, the yield strength YS of the electric resistance welded steel pipe for line pipe exceeded 540 MPa, and the tensile strength TS exceeded 625 MPa, which were too high.

In test number 16, the heating temperature exceeded 1200°C. Therefore, the temperature T ₀ at the exit side of the final stand in rough rolling became too high, and the time t ₀ from immediately after the end of rough rolling to the start of finish rolling exceeded F2. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio exceeded 20%. Furthermore, the {100} density was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In Test No. 17, the heating temperature was less than 1060°C. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio exceeded 20%. Furthermore, the {100} density was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In test number 18, the finish rolling temperature was too high. Therefore, the {100} integration degree was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In test number 19, the finish rolling temperature was too low. Therefore, the coarse crystal grain ratio exceeded 20%. Furthermore, the {100} density was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In test number 20, V1 was less than 5°C/s. Therefore, the crystal grains became coarse, the effective crystal grain size exceeded 15 μm, and the coarse crystal grain ratio exceeded 20%. Therefore, the {100} integration degree was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

In test number 21, the time t ₀ from immediately after the end of rough rolling to the start of finish rolling exceeded F2. Therefore, the {100} integration degree was less than 1.50. As a result, the DWTT guarantee temperature was higher than -30°C and the low temperature toughness was low.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (8)

in % by mass,
C: 0.010 to 0.060%,
Si: 0.05 to 0.30%,
Mn: 0.50-2.00%,
P: 0 to 0.030%,
S: 0 to 0.0100%,
Al: 0.010-0.035%,
N: 0.0010 to 0.0080%,
Nb: 0.010 to 0.080%,
Ti: 0.005 to 0.030%,
Ni: 0.001 to 0.50%,
Mo: 0.05-0.30%,
O: 0 to 0.0030%,
Ca: 0 to 0.0050%,
V: 0 to 0.100%,
Cr: 0 to 0.30%,
Cu: 0-0.30%,
Mg: 0 to 0.0050%, and
Rare earth element: containing 0 to 0.0100%, the balance being Fe and impurities, having a chemical composition that satisfies formula (1),
In the structure at the center of the thickness, the ferrite fraction is 60 to 90%,
In orientation measurement at each measurement step of 0.3 μm, when the position where the orientation difference between adjacent measurement points exceeds 15° is defined as the grain boundary, and the region surrounded by the grain boundary is defined as the crystal grain, the area average grain The effective crystal grain size , which is the diameter, is 15 μm or less, and the coarse crystal grain ratio, which is the area ratio of the crystal grains having a crystal grain size of 20 μm or more, is 20% or less,
When the surface perpendicular to the rolling direction is the RD surface, the rolling surface is the ND surface, and the RD surface and the surface perpendicular to the ND surface are the TD surface, the angle formed with the RD surface is 45°, and A steel material for line pipes, wherein in a specific plane forming an angle of 45° with a TD plane, the degree of accumulation of {100} planes is 1.50 to 2.50.
0.300≦C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/3+Nb/3≦0.380 (1)
Here, the content (% by mass) of the corresponding element is substituted for each element symbol in formula (1). The steel material for line pipe according to claim 1,
The chemical composition, in mass %,
A steel material for line pipes containing Ca: more than 0 to 0.0050%. The steel material for line pipes according to claim 1 or 2,
The chemical composition, in mass %,
V: more than 0 to 0.100%,
Cr: more than 0 to 0.30%, and
A steel material for line pipes containing one or more selected from the group consisting of Cu: more than 0 to 0.30%. The steel material for line pipes according to any one of claims 1 to 3,
The chemical composition, in mass %,
Mg: more than 0 to 0.0050%, and
A steel material for line pipes, containing one or more selected from the group consisting of rare earth elements: more than 0 to 0.0100%. The steel material for line pipes according to any one of claims 1 to 4,
The steel material for line pipes is a steel material for line pipes, which is a hot-rolled steel sheet for line pipes. The steel material for line pipes according to any one of claims 1 to 4,
The steel material for line pipes is a steel material for line pipes, which is an electric resistance welded steel pipe for line pipes. The electric resistance welded steel pipe for line pipe according to claim 6,
An electric resistance welded steel pipe for line pipe, having a yield strength of 450 to 540 MPa in the pipe axial direction and a tensile strength of 510 to 625 MPa in the pipe axial direction. The electric resistance welded steel pipe for line pipe according to claim 7,
An electric resistance welded steel pipe for line pipe, having a wall thickness of 12 to 25 mm and an outer diameter of 304.8 to 660.4 mm.

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